A post-detection selection diversity receiver is one which uses at least two antennas for improved reception of radio frequency (RF) signals. Each antenna and corresponding receiver together generate a received signal. From the at least two received signals available, one of the signals is selected. Generally, there are two types of post-detection selection diversity receivers available today. In the first type, the selection of one the received signals is dependent upon the received signal strength (RSS) of the signals. This type of diversity receiver is known as the level comparison diversity technique. The selection in the second type is dependent upon phase error estimates. This second type of post-detection selection diversity receiver is known as the phase likelihood comparison technique.
In the level comparison diversity technique, the RSS of each received signal is detected and compared to the others. The received signal having the highest RSS value is selected for use by the radiotelephone. This simple system is relatively effective in a Rayleigh fading environment and superior to a system having a single antenna and receiver. However, the level comparison diversity technique does not offer any advantage over a single antenna system in a static environment.
The phase likelihood comparison technique utilizes a phase error estimate from each of the received signals, selecting the signal with the smallest estimated phase error. This embodiment is the subject of a Japanese patent application number HEI2-253727, entitled "Diversity Reception Circuit", assignee Nippon Telegram and Telephone Corporation, filed Mar. 28, 1989. First, the phase of each of the received signals is detected. Next, the phase is plotted onto an appropriate signal constellation. The constellation contains symbol decision points. A phase error vector is created between the plotted phase vector and the nearest symbol decision point. The phase error vector is created for each received signal. The received signal having the smallest phase error vector is used by the radiotelephone for further processing. The other received signals are ignored. Note that the phase likelihood comparison technique does not utilize the RSS information.
An enhancement to the phase likelihood comparison technique is discussed in the paper, All Digital Adaptive Carrier Tracking Coherent Demodulator, Shigeki Saito, Harui Yamamoto, Yasushi Yamao. NTT Radio Communication System Laboritories, February, 1991. The enhancement combines the use of RSS information and phase error information for a two branch diversity receiver. The RSS of the received signals are compared to each other. If the difference between the RSS values exceeds a predetermined threshold, then the received signal with the larger RSS value is used. If the difference between the RSS values is less than the predetermined threshold, then the phase error information is used in the aforementioned manner to make the selection. This enhancement offers improved performance over the straight phase likelihood comparison technique; however, further improvement in performance is desirable. Furthermore, the requirement to generate a phase error estimate restricts the phase likelihood comparison technique to digitally phase modulated signals.
The market today is forcing manufacturers of radiotelephones to reduce the size, weight and power consumption of the radiotelephone while increasing the performance criteria. Therefore, a need exists for a simple diversity receiver circuit which has improved received signal performance when compared to the conventions available today.